It is appropriate to specify beforehand that, although in the following description reference will be made for simplicity mainly to cold-chamber pressure die casting, this should not however be understood as a limiting factor, since the present invention is also applicable to, unless specifically incompatible with, other types of pressure die casting processes (e.g. hot-chamber die casting) for metallic or non-metallic materials.
The cold-chamber pressure die casting process has been known for a long time, and therefore it will not be described in detail below, with the exception of what is strictly needed in order to understand the invention. For further information, reference should be made to the numerous technical and scientific publications on this matter.
In this process, molten metal is poured into a container having a cylindrical inner cavity, in which the metal is pushed by a moving piston towards an axial outlet, thereby being injected into a die containing the mould of the part to be cast.
This type of process is mostly used for producing parts made of aluminium-based light alloys, but its field of application has been recently extended to magnesium as well; the temperatures involved may reach quite high values (over 400-500° C.), and therefore piston cooling is an important factor for the proper execution of the production process.
According to the current state of the art, in these applications the piston is cooled by a liquid which is delivered to the most thermally stressed region, i.e. the piston head, which comes directly in contact with the molten metal, and is then evacuated along an inverse path.
In particular, the liquid flows into an axial duct within the support on which the piston is mounted, leading to the piston head; the liquid is spread onto the inner wall of the piston head through radial channels provided at the support end.
The coolant flow is thus distributed in a sunburst pattern and is then collected into a circular channel encircling the piston support, from which it finally returns to the support's axial portion to be evacuated.
Some examples of pistons cooled in this manner are described in European patent application EP 423 413 published on Apr. 24, 1991 and in International patent application PCT/IT2007/000255 published on Oct. 18, 2007.
While from a general viewpoint the cooling systems known in the art are considered to be reliable because they have been tested for a long time, the higher temperatures nowadays involved in pressure die casting processes, as aforementioned, give rise to the need of improving the efficiency of the thermal exchange between the piston and the coolant.
As a matter of fact, casting magnesium and its alloys makes the piston become very hot: it follows that, in order to remove more heat, there is no other solution than to act upon the thermal exchange area lapped by the coolant, i.e. to increase the piston dimensions.
However, this is not always feasible, because it would also require changes to the container in which the piston slides, so that de facto this solution is not applicable to existing pressure die casting fixtures, which otherwise should be replaced, involving high costs.
The technical problem at the basis of the present invention is therefore to improve the above-described state of the art.
In other words, the problem is to provide a pressure die casting piston which is cooled with greater efficiency than possible through the prior art.
A piston having the same diameter as the existing ones can thus be manufactured which, all other conditions being equal (coolant flow rate, wall length, etc.), ensures better performance because it is cooled more effectively.
The idea which provides a solution to the above-mentioned technical problem is to let the coolant flow within the piston wall: in this manner, the heat is removed directly from within the latter, thus increasing the thermal exchange.
The better piston cooling allows to increase the number of die casting cycles while still keeping the piston temperature under preset values ensuring the proper operation of the machine.
As a result, the productivity of the die casting equipment is increased as well, with evident advantages from an industrial point of view.
The aforementioned technical problem is solved by a piston having the features set out in the appended claims.
Said features and the advantages thereof will become more apparent from the following description of an embodiment of the piston according to the invention referring to the annexed drawings.